Astrophile: Wounded galaxy is crux of cosmic whodunnit

Astrophile is our weekly column covering curious cosmic objects, from within the solar system to the furthest reaches of the multiverse

Object type: GalaxyDistance: 300 million light-years

A wounded galaxy is bleeding an astonishingly long trail of gas into space, yet there is no obvious perpetrator.

Frustrating even the most resolute of cosmic detectives, this crime scene illustrates the dangers that galaxies face – the loss of such a large volume of gas can eventually kill. It also suggests we may need to revise our theories of how and where they can be wounded.

Space is a dangerous place for galaxies. Collisions with other galaxies, near misses and other forms of violence can dramatically alter or even destroy them.

Hydrogen trail

Even when disturbances do not destroy galaxies, they can strip them of the gas they need to produce new generations of stars and planets, effectively sterilising them.

The latest known victim of such intergalactic violence is called FGC 1287. It appears perfectly healthy in visible light, which shows only its stars, but new radio evidence from the Very Large Array in New Mexico reveal that it is severely wounded, bleeding a vast trail of gas into space.

This galactic tail is one of the longest ever seen, stretching across 800,000 light years. "When we looked at the data, we were amazed," says Tom Scott of the Andalucia Institute of Astrophysics in Granada, Spain, who led the study.

The tail is made of billions of suns' worth of hydrogen gas that has somehow bled from FGC 1287 – more gas than remains in the galaxy itself. But the cause of the galactic haemorrhage is hard to pin down.

Bloody footprints

"FGC 1287 is completely different from any case we have seen before," says team member Luca Cortese of the European Southern Observatory in Garching, Germany.

One possible explanation is the galaxy's location at the edge of a cluster of other galaxies. When a galaxy falls into a cluster, it often has gas stripped away by "headwinds" created as the galaxy moves through the cluster's own gas.

But X-ray observations of the cluster's gas suggest it is too thin at FGC 1287's location to make the galaxy feel much of a wind, says the team.

A near miss with another galaxy might be a better lead and could explain why the galaxy's stars are not in disarray. In a brief encounter at high speed, the second galaxy's gravity could have pulled away loosely bound gas without disturbing its stars, which are more tightly bound to the galaxy by gravity. Intriguingly, like bloody footprints leaving a crime scene, the gas trail points in the direction of another galaxy.

Violent mystery

The case is not closed, however, because the suspect galaxy looks much less massive and shows no sign of trauma itself. It is not clear how it could have made such a large impact on the heavier galaxy without being wounded itself.

That leaves FGC 1287's condition a mystery. Whatever the solution, the team say the case suggests we have a lot to learn about how violence befalls galaxies. Perhaps FGC 1287 is enduring stronger winds than expected, or another galaxy not obviously connected to it will turn out to be its attacker.

"This discovery might open a new chapter in our understanding of environmental effects on galaxy evolution," says team member Elias Brinks of the University of Hertfordshire, UK.

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